The present invention relates to a laser/arc hybrid welding process for ferritic steels using a filler wire and to a weld obtained with such a hybrid process, that is to say a welding process that simultaneously employs an electric arc and a laser beam, these being combined with each other.
Welds produced by laser/arc hybrid welding on ferritic steels, such as C-Mn steels defined by the EN 10025 standard, microalloyed steels defined by the EN 10113 standard or else “quenched and tempered” steels according to the EN 10137 standard, usually have poor resilience properties, or more generally poor low-temperature toughness, in the melted metal, and also hardness values in this same zone that are very much higher than those of the base metals.
These mediocre metallurgical properties very greatly limit the extension of laser/arc hybrid welding to certain fields in industry, in particular in the naval construction field, the field of the manufacture and laying of pipes for transporting oil products, the offshore field, etc.
This problem results from the fact that such steels have been chemically balanced in order to give them the intended mechanical properties, taking into account their production process, that is to say the subsequent rolling and cooling conditions, or else the heat treatment that they undergo during their manufacture, for example in the form of sheet or plate or tube.
In fact, the mechanical properties of a steel result, in part, from its chemical composition but for a larger part from its microstructure.
The microstructure of a steel, and likewise of a weld, that is to say of the melted metal formed by deposited metal and base metal that has melted during production of the weld, develops from the high-temperature austenitic state during cooling down to the ambient temperature.
Consequently, for a given chemical composition, this microstructure and consequently the mechanical properties of the steel (or of the weld) depend on the cooling conditions.
If we consider for example a steel containing about 0.12% carbon by weight, with a low cooling rate, its structure is essentially composed of ferrite, that is to say iron atoms stacked in a body-centred cubic crystallographic structure, and of a small percentage (typically around 13%) of pearlite, that is to say alternating lamellae of ferrite and cementite, which is iron carbide Fe3C containing 6.66% carbon. Its Vickers hardness is then about 130 and its tensile strength is around 400 to 500 MPa.
However, this same steel will have a martensitic structure, that is to say a supersaturated solid solution of carbon in the body-centred cubic iron, and a Vickers hardness of around 400, while its tensile strength will be 1300 to 1400 MPa if it undergoes extremely rapid cooling from the austenitic (high temperature) state.
For cooling rates between these two extremes, mixed structures, composed of martensite, lower bainite, upper bainite and ferrite+pearlite, to which intermediate mechanical properties correspond, will develop.
Continuous cooling transformation (or CCT) diagrams, which are well known to metallurgists, indicate the various microstructures that develop and also hardnesses that correspond thereto depending on the cooling rate for a given steel and the standard austenization conditions for this steel, namely the temperature (generally 50° C. above the point for complete transformation to austenite) and the austenizing time (generally 30 minutes).
Such diagrams also show that, for a given steel, the difference in mechanical properties between the martensitic structure and the ferrite+pearlite structure is greater the higher its carbon content. They also show, if the diagrams for steels of various compositions are compared, that the cooling rates that generate the abovementioned various microstructures depend on all of the alloying elements of the steel.
This is because all the alloying elements have an impact on hardenability, that is to say the ability of a steel to acquire a fully martensitic structure, and therefore also on the critical quenching rate, which is the minimum cooling rate from the austenitic state that allows a 100% martensitic structure to be obtained.
The carbon content, in addition to affecting the hardenability, also determines the mechanical properties of the various structures.
Laser/arc hybrid welding processes, because of the high power density associated with them and the high welding speeds that they allow to be achieved, which are often higher than in laser welding alone, result in very rapid cooling rates.
It therefore follows that, with ferritic steels, the microstructure of the weld is very different from that of the base metal, which results, in this zone, in hardness and tensile properties that are much higher than those of assembled steels, but also in a ductility and a toughness of the weld that are too low for many applications.
This effect may be alleviated by the addition of a filler metal in the form of a “cold” wire, that is to say a welding wire paid out in the sheet joint plane immediately upstream of the impact of the laser beam and of the arc employed in laser/TIG or plasma hybrid welding, or in the form of a consumable electrode wire when the process is a laser/MIG or MAG hybrid process.
In fact, by proceeding in this manner, the aim is to adjust the hardenability of the melted metal usually by reducing its content of alloying elements relative to the base metal(s), but also, in the case of very mild steels, that is to say those having a yield strength of less than 240 or 280 MPa, by increasing it.
In both cases, the aim is to adjust the hardenability of the melted metal so that, under the effect of the thermal cycle caused by the hybrid welding, it develops a less brittle microstructure.
However, it is often insufficient to proceed in this manner since, with hybrid laser welding processes, the proportion of filler metal in the melted metal is very often around 20% by weight and very rarely exceeds 40% by weight, which means that, even when using commercially available wires containing the least amount of alloying elements, that is to say wires containing 0.5% manganese by weight for example, the hardenability of the melted metal cannot be lowered sufficiently to prevent the formation of hard and brittle structures in the case of steels which, without this addition, already result in a hard and brittle structure.
Moreover, in the case of very mild steels for which it may be desirable to increase the hardenability, in order to avoid the formation of a coarse and brittle structure, the addition of a wire having a higher content of alloying elements than the metal to be welded, so as to obtain a finer structure in the melted metal, does not appear to be a satisfactory solution either, since this refinement of the structure is accompanied by a substantial increase in the hardness and thereby results only in a small reduction in brittleness.
The problem that therefore arises is how to improve laser/arc hybrid welding processes so as to be able to obtain welds whose mictrostructure is virtually free of hard brittle microconstituents, that is to say those having improved properties in terms of resilience, and more particularly in terms of toughness, and also tensile properties compared with those of the base metals, especially an increased elongation, a lower tensile strength and a lower yield strength, while still remaining higher than those of assembled materials.